(1) Field of the Invention
The present invention relates to digital telephone switching systems and more particularly to a clock synchronization circuit for synchronization of internal clock signals to those received externally.
(2) Description of the Prior Art
Communication systems have long been connected to distant switching systems by means of a digital communication link. Some of these communication systems operate under control of an asynchronous clock circuit and employ complex circuitry for monitoring and deciphering the digital information received over the communication link.
More modern communication systems use a clock circuit which operates in synchronism with the digital data received over the communication link. These newer systems typically employ tank circuits for monitoring the presence of the incoming digital signal, and use counter circuits for monitoring the frequency of these signals.
However these arrangements are unnecessarily complex, bulky and expensive. These arrangements are also insufficiently responsive for detection of failure of the incoming digital signal and they operate only to provide frequency synchronization rather than phase synchronization between the incoming digital signal and the internally generated clock signals.
Accordingly, it is the object of the present invention to provide a small, low cost and highly responsive clock synchronization circuit which provides not only frequency synchronization but also phase synchronization between the incoming digital signal and the internally generated clock signals.